Area Forecast Discussion
Issued by NWS Austin/San Antonio, TX

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FXUS64 KEWX 200339 AAA

Area Forecast Discussion...UPDATED
National Weather Service Austin/San Antonio TX
939 PM CST Mon Feb 19 2018

First set of storms coming across the Seminole Canyon State Park in
west Val Verde County as of 930 PM this evening. More storms are
expected to develop tonight into the overnight hours as an upper
level short wave moves from northeast Mexico into Val Verde County
and southern Edwards Plateau overnight. There is enough instability
with surface to 6 km shear values of 50 to 60 knots for storms to
quickly develop and become strong to marginally severe. The main
hazards as these storms develop will be hail up to one inch in
diameter and thunderstorm wind gusts of 60 mph. Some quick and heavy
downpours could end up with one inch of rainfall accumulations.


.PREV DISCUSSION... /ISSUED 519 PM CST Mon Feb 19 2018/

Scattered showers have developed southwest of the San Antonio sites
and will begin these terminals with VCSH. A more robust complex of
showers and thunderstorms will develop west of DRT this evening and
will bring TSRA to DRT between 3z and 7z. This system will possibly
impact the I-35 sites around 9-15z and will mention VCTS for these
sites because of the lower probability. MVFR/IFR conditions are
expected again tonight with slow improvement anticipated in the
afternoon. Additional rain chances are expected tomorrow afternoon
with higher chances beyond the current TAF cycle.

PREV DISCUSSION... /ISSUED 338 PM CST Mon Feb 19 2018/

SHORT TERM (Tonight through Tuesday Night)...
A very active weather pattern through the short term forecast, with
first a chance of strong to severe storms tonight across the
northwest CWA and then a chance for locally heavy rainfall across a
good portion of the CWA Tuesday night into Wednesday morning. An
active sub-tropical branch of the jet stream will send multiple
disturbances through the region over the next 36 hours.

The first weak disturbance is currently generating a band of light
rain near the I-35 corridor, which continue to lift northeast through
the northern and eastern CWA through the remainder of the afternoon.
Forcing from a more potent upstream disturbance coming out of Mexico
and de-stabilization taking place along the Rio Grande, where
clearing has taken place, will generate convection across the higher
terrain of Mexico and the southern Edwards Plateau this evening.
Several HREF members, the HRRR, and TT WRF indicate a semi-organized
complex developing across the northwest CWA later this evening and
lifting northeast into the Hill Country overnight, as the convection
encounters a developing 40 kt low level jet and theta-e ridge. Deep
layer shear values of 50-60 kts, MUCAPE values of 1000-1500 J/kg, and
mid level lapse rates of around 6 deg C/km are supportive of some
strong to severe storms, mainly near and north of a Eagle Pass to
Medina, to Burnet line, where current SPC Marginal Day 1 risk is.
Main threats will be hail as well damaging straight-line winds,
should the convection become more organized. Can`t rule out a quick
1-2 inches of rainfall in some spots, but should be progressive
enough to limit potential for widespread flooding.

Farther east into the I-35 corridor, showers should be developing
beneath the inversion late in the night and into the Tuesday morning.
Can`t rule out some isolated elevated thunderstorms further east.
Majority of the guidances decays the semio-organized storms Tuesday
morning before reaching the I-35 corridor, however should note that
recent HRRR runs are trending closer to the I-35 corridor with the
convection. There could be some re-development of deeper convection
Tuesday afternoon across portions of the northern and eastern CWA.

TT WRF and GFS runs are indicating strong upward vertical motion then
developing late Tuesday night ahead of another impulse in the
southwest flow aloft. Recent runs of both models are indicating
widespread convection developing out of Mexico into the southwest CWA
late Tuesday night and then into a broad low level convergence area,
ahead of a cold front, across the Hill Country and I-35 corridor
overnight Tuesday into Wednesday morning. Forecast soundings are
favorable for locally heavy rainfall and training over some
locations. Forecast precipitable water values are forecast to be an
impressive 1.5-1.8" across central and eastern areas of the CWA,
which would be climatological maxes for this time of year. Should
note that run to run consistency, as well as NAM12 and ECMWF, are
adding to some uncertainty in exact placement of heavier banding, but
in general ingredients are coming into place for the potential for
pockets of heavy rainfall somewhere across the eastern 2/3rds of the
CWA Tuesday night through Wendsday morning. We have included a
mention of locally heavy rainfall into the forecast and will continue
to advertise 1-3 inches with isolated pockets up to 5 inches,
although as mentioned the pockets are difficult to determine at this
time. Day 2 and Day 3 Excessive Rainfall Outlooks from WPC continue
with a Slight Risk of exceeding flash flood guidance near and north
of I-10. A Flash Flood Watch could be considered tomorrow as forecast
confidence in placement of heavier banding increases.

LONG TERM (Wednesday through Monday)...
The aforementioned cold front eventually works through the CWA during
the day on Wednesday and undercuts the deeper convection. However
good chances for post frontal showers and isolated thunderstorms will
continue behind the front into Wednesday afternoon with falling
temperatures. Much cooler on Thursday with continued chances for
showers. The front works back north as a warm front Thursday night
and Friday, with warmer temperatures expected on Friday. Another cold
front is expected to move through the area on Saturday providing a
quick chance of showers and slightly cooler temperatures over the


Austin Camp Mabry              67  72  61  66  45 /  50  80 100 100  50
Austin Bergstrom Intl Airport  66  73  62  66  46 /  40  70 100 100  50
New Braunfels Muni Airport     66  73  63  69  47 /  40  70 100 100  50
Burnet Muni Airport            65  72  57  59  41 /  60  80  90  90  40
Del Rio Intl Airport           63  81  61  72  48 /  50  10  50  70  30
Georgetown Muni Airport        66  71  58  62  43 /  60  80  90  90  50
Hondo Muni Airport             67  78  64  71  49 /  50  50  90  90  40
San Marcos Muni Airport        66  73  62  68  46 /  40  70 100 100  60
La Grange - Fayette Regional   68  74  66  72  51 /  20  70  80  90  70
San Antonio Intl Airport       67  75  63  70  48 /  40  60 100 100  50
Stinson Muni Airport           67  76  64  73  50 /  40  60  90 100  50




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